According to a study published Monday in the Proceedings of the National Academy of Sciences, the fossil may be the oldest and most detailed example of a central nervous system yet identified, with even individual nerves -- rarely preserved soft tissue -- visible enough to study.
Most fossil specimens are the remains of teeth (the hardest part of the body) or bones. Soft tissues, such as nerves, are much more likely to decay over time. So studying the long evolution of nervous systems can be a daunting prospect.
The fossil described in the new paper, which was found in southern China, is a rare exception.
Researchers from Yunnan University had to take special precautions to maintain the integrity of the delicate system.
"Using a fine needle and a steady hand, they chipped away parts of the rock to reveal the preserved internal features; they only needed a bit of nerve cord sticking out to have a good idea of where to continue excavating in the fossil," study co-author Javier Ortega-Hernández of the University of Cambridge told The Washington Post in an email. "As usually happens with amazing discoveries, when I first saw the material it took me a bit of time to make sense of what I was looking at. After a little while, however, excitement kicked in after realizing that not only was this an exquisitely preserved nerve cord, but also that it has impossibly thin individual nerves sticking out from it!" he wrote.
That nerve cord — analogous to the spinal cord found in modern in vertebrates — ran through its entire body. Bead-like clusters of nerve tissue called ganglia (which act like "mini-brains along the nerve cord," according to Ortega-Hernández) each controlled a single pair of the animal's many legs.
Scientists have seen animals of similar age with preserved ganglia before, which Ortega-Hernández believes is the result of the fatty content of that nerve tissue — some of those chemicals may make it more prone to fossilization than other soft stuff in the body is. But Chengjiangocaris kunmingensis also shows a few dozen delicate, tiny individual nerve fibers sprouting out from the nerve cord and ganglia.
That may be an important clue in the evolutionary mystery.
"By contrast, arthropods of today only have the ganglia, but a very restricted number of the lateral nerves," he said. "The only living group today where scientists have found dozens of individual nerve fibers structured like these are priapulids (penis worms) and onychophorans (velvet worms), which are cousins of arthropods.
"Putting all of this together, the new fossils of C. kunmingensis allow us to recognize that the evolution of the nervous system in living arthropods involved the loss of some of these nerves, and that their presence in the fossil and the velvet worms is a very ancestral feature," Ortega-Hernández explained.
Ironically, while the shrimp-like creature's nervous system has revealed itself to researchers, its legs remain mysterious. In an email to The Post, corresponding author Xi-guang Zhang of Yunnan University explained that despite the unprecedented nerve preservation, the specimen's legs were too far gone to make an accurate count.
"We know C. kunmingensis has twenty-six limb-bearing trunk segments," Zhang wrote. Of these, the scientists are fairly certain that the front five segments each had one pair of identical legs. It's possible that the rear three or four segments were totally limbless, and other segments could have had two or even four pairs of legs apiece.
"So an individual possesses several tens of paired limbs," Zhang said, "but because of incomplete preservation we still do not know exactly how many pairs it would have had."
The researchers will need to find even more of these intact nervous systems if they want to complete the evolutionary puzzle. But that could prove difficult: After all, the specimen described in the study was a rare bird (erm, rare ancient shrimp-y thing).
"We have to bear in mind that each nerve strand is about 10 times thinner than a human hair," Ortega-Hernández said, "so being able to recognize this level of neurological detail is simply amazing."
This article was originally published on Feb. 29. It has been updated.